The resolution and field of view obtainable in electron beam instruments such as scanning electron microscopes, electron beam microprobes, electron beam lithographic machines and oscilloscopes is limited by third order aberrations of the optical system. An electron beam instrument is one which produces an electron beam directed along an axis, has a focusing lens and means for scanning the focused beam. These aberrations are of eight types: isotropic and anisotropic distortion, curvature of field, isotropic and anisotropic astigmatism, isotropic and anisotropic coma, and spherical aberration. For an image point on the optical axis of the system, only spherical aberration occurs. As the electron beam is focused onto image points farther off axis, the remaining seven aberrations become significant in determining the attainable focused spot size and the degree of distortion present in the image.
Several of these optical aberrations are easily corrected with commonly known techniques. The only aberrations which limit field of view for which some sort of simultaneous correction has not been provided are isotropic and anisotropic coma. As is commonly known, proper placement of the beam defining aperture enables isotropic coma to be cancelled, even in nonscanning electron beam instruments such as the conventional electron microscope. There has been, however, no means of completely correcting the anisotropic coma aberration. Further, the simultaneous correction of anisotropic and isotropic while minimizing curvature of field has not been provided.
It is therefore an object of this invention to provide an electron beam system having simultaneous correction of both isotropic and anisotropic coma.
Another object of this invention is to provide a double delfection scanning system for an electron beam instrument which provides simultaneous correction of isotropic and anisotropic coma.